add app.error(fn). Closes #4

lib/application.js

@@ -139,6 +139,17 @@ app.callback = function(){
   }
 };
 
+/**
+ * Set the application-level error handling `fn`.
+ *
+ * @param {Function} fn
+ * @api public
+ */
+
+app.error = function(fn){
+  this.onerror = fn;
+};
+
 /**
  * Default error handler.
  *